Listeria-based cancer vaccines that segregate immunogenicity from toxicity

The facultative intracellular bacterium Listeria monocytogenes is being developed as a cancer vaccine platform because of its ability to induce potent innate and adaptive immunity. For successful clinical application, it is essential to develop a Listeria platform strain that is safe yet retains the potency of vaccines based on wild-type bacteria. Here, we report the development of a recombinant live-attenuated vaccine platform strain that retains the potency of the fully virulent pathogen, combined with a >1,000-fold reduction in toxicity, as compared with wild-type Listeria. By selectively deleting two virulence factors, ActA (triangleactA) and Internalin B (triangleinlB), the immunopotency of Listeria was maintained and its toxicity was diminished in vivo, largely by blocking the direct internalin B-mediated infection of nonphagocytic cells, such as hepatocytes, and the indirect ActA-mediated infection by cell-to-cell spread from adjacent phagocytic cells. In contrast, infection of phagocytic cells was not affected, leaving intact the ability of Listeria to stimulate innate immunity and to induce antigen-specific cellular responses. Listeria triangleactA/triangleinlB-based vaccines were rapidly cleared from mice after immunization and induced potent and durable effector and memory T-cell responses with no measurable liver toxicity. Therapeutic vaccination of BALB/c mice bearing murine CT26 colon tumor lung metastases or palpable s.c. tumors (>100 mm(3)) with recombinant Listeria triangleactA/triangleinlB expressing an endogenous tumor antigen resulted in breaking of self-tolerance and long-term survival. We propose that recombinant Listeria triangleactA/triangleinlB expressing human tumor-associated antigens represents an attractive therapeutic strategy for further development and testing in human clinical trials.